In recent years, power saving in information processing devices has been an important issue and there has also been a strong demand for power saving in a bus that is a signal transmission path in an information processing device. As one attempt to realize power saving of a bus, the PCI Express, which is an I/O bus widely used at present, has a mode called a sleep mode. The sleep mode is a mode in which a bus is brought into a stopped state in such a case where the bus has not been used for a certain period.
As another attempt to reduce power consumption in a bus, changing the transfer mode from the double data rate (DDR) to the single data rate (SDR) depending on the situation can be conceived. A known technique of changing the transfer mode is, for example, a technique of providing a signal line for transmitting a signal indicating whether to transmit data at the DDR or at the SDR and switching a signal to be transmitted via the signal line to change the transfer mode.
However, the method of realizing power saving using a sleep mode can only be applied to limited cases because it is necessary that a bus be not used at all for a certain period. For example, when a bus is continuously used, the bus cannot be brought into the sleep mode because the data cannot be transferred via the bus in the sleep mode.
On the other hand, the method of changing the transfer mode from the DDR to the SDR can be applied even when a bus is continuously used if the use rate of the bus is low. However, if a signal line for transmitting a signal specifying the transfer mode is provided separately from a signal line for transmitting data signals as in the aforementioned related technology, this causes disadvantageous effects such as complicated configuration of the devices and increase in the cost.    Patent Document: Japanese Laid-open Patent Publication No. 2001-118325